This presentation focuses on crack nucleation in polycrystalline freshwater and saline ice. A historical review of the plausible nucleation mechanisms is provided, followed by a discussion of outstanding research challenges.

Since the late 1990s, several nucleation mechanisms have been investigated. These include: (i) dislocation pileup against obstacles such as grain boundaries, (ii) grain boundary sliding leading to displacement incompatibility at triple junctions, and (iii) elastic anisotropy of the hexagonal ice crystals giving rise to microstructural stresses which can nucleate cracks from precursors. For saline and sea ice, pressurized brine pockets are stress concentrators and likely crack nucleation sites. The basal slip system is dominant in ice, and cracks may nucleate to relieve the strain heterogeneity arising from the strong plastic anisotropy of ice deformation. Furthermore, the interaction of various crack nucleation mechanisms under different conditions, e.g., temperature, grain size and texture, and the effect of time and loading rate on crack nucleation in viscoelastic ice, are issues that have received little attention. These are therefore research challenges that can be investigated in the future.

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